#include <stdio.h>
#include <math.h>
#define PI 3.1415926535

int main(void) {
    double xa, ya, xb, yb;
    scanf("%lf%lf%lf%lf", &xa, &ya, &xb, &yb);
    printf("%g\n", sqrt(pow(xa - xb, 2.0) + pow(ya - yb, 2.0)));
    return 0;
}
